There has been known, by Patent Document 1 for example, in a permanent magnet rotating machine including a stator and a rotor, an interior permanent magnet (IPM) rotating machine configured such that reluctance torque can also be used in addition to magnet torque by embedding a permanent magnet in a rotor core. In this kind of rotating machine, the rotor core is located opposing to the stator having a plurality of magnetic poles with an air gap in between. Also, slots are formed at equally spaced circumferential positions of the rotor core, and the permanent magnet is inserted into the respective slots.
In addition, there has also been known, in a permanent magnet rotating machine, a surface permanent magnet (SPM) rotating machine in which a cylindrical (also referred as a ring-shaped) permanent magnet is located on a surface of a rotor core. In this kind of rotating machine, the permanent magnet is located opposing to the stator having a plurality of magnetic poles with an air gap in between, and thus, strong magnetism, which the permanent magnet has, can efficiently be used.
In such kind of motors, when the rotor is rotated, a magnetic flux passing through the permanent magnet incorporated in the rotor core changes, and an eddy current is generated inside the permanent magnet to counteract the change in the magnetic flux. When the eddy current is generated inside the permanent magnet, a temperature of the permanent magnet reaches up to a thermal demagnetization region, and magnetic property may be deteriorated. Thus, an eddy-current loss due to such eddy current is desired to be reduced.
As a means for reducing the eddy-current loss, dividing the permanent magnet into a plurality of smaller permanent magnet pieces has been found to be effective. By dividing the permanent magnet into a plurality of permanent magnet pieces, a flow channel of the eddy current flowing through each of permanent magnet pieces becomes longer and a density of eddy current of each of the permanent magnet pieces decreases, thus, the eddy-current loss of the entire permanent magnet is reduced.
As a proposal for such permanent magnet, there is a technique disclosed in Patent Document 2, for example. This technique relates to a permanent magnet which is equally divided in a rotational axis direction or a circumferential direction of a rotor. However, merely dividing one permanent magnet equally into a plurality of permanent magnet pieces may result in dividing not only parts effective for reducing the eddy-current loss, but also parts which have less reduction effect of eddy-current loss. In this case, since parts, which are not necessary to be divided, are divided, not only the eddy-current loss cannot be reduced effectively, but also an entire performance of the magnet could be deteriorated.
As a technique to deal with such problem, a method for positively defining positions to divide a permanent magnet is proposed for a purpose of effectively reducing an eddy-current loss of the permanent magnet.
Patent Document 3 proposes a technique for, in a permanent magnet disposed in a movable member of a rotating machine, reducing an eddy-current loss by dividing the permanent magnet into a plurality of magnets having different width. Such width is defined corresponding to a proportion of change in a magnetic flux density inside the permanent magnet. Such width is made narrower as the proportion of change in the magnetic flux density increases and is made wider as the proportion of the change decreases, and specifically, it is defined such that the eddy-current loss generated in each of the divided magnets becomes substantially equal. Such width of the permanent magnet located at a tip in a rotational direction may be made narrower than other parts.
Patent Document 4 proposes a technique for reducing an eddy-current loss during a flux weakening control in an IPM motor by narrowing a width of a permanent magnet at a rear part in a rotational direction of a rotor. In this technique, such width is defined such that the eddy-current loss at each of permanent magnet pieces becomes equal, and the eddy-current loss is calculated based on a variation range of a magnetic flux density.